Reduced fouling from the convection section of a cracker

ABSTRACT

Crackers for hydrocarbon such as naphtha and C2-C4 paraffins contain a radiant section and a convection section. The exhaust gases leaving the radiant section pass through the convection. Generally fouling from the convection section was low relative to fouling (e.g., coke build up) in the radiant section. With improved metallurgy and operating conditions, the time between decokes of the radiant section has increased and now there is a need to reduce fouling from the convection section. This may be achieved by using stainless steel, and particularly high nickel, high chrome stainless steel in the passes in the convection section.

FIELD

The present disclosure relates to an initial heat treatment of hydrocarbons at temperatures up to about 560° C. in for example the convection section of a heater.

Typically the hydrocarbon is then fed to the radiant section of a furnace where it is further cracked at temperatures typically between 800 and 950° C. Downstream of the radiant section is a transfer line leading to a quench section. To extend the operation of the process it is desirable to minimize coke build up from the convection section.

Initially, when the tubes in a radiant section were not treated or there was no careful control over the cracking in the radiant section, the time between off line operation to “burn” out the accumulated coke, may have ranged from about 30 to about 65 days. Now, the run time for the radiant section of the furnaces can be extended to 100 days or more. To achieve these longer run times, more effort is being focused on the reducing coke formation from the convection section.

BACKGROUND

U.S. Pat. No. 4,986,222 issued Jan. 22, 1991 to Pickell et al., assigned to Amoco Corporation. The patent relates to furnaces for oil refiners or petrochemical plants. The patent discloses thermal treatment for high sulphur naphtha and hydrogen to produce gasoline and aromatic feedstocks, not olefins. At claim 8 and Col. 5 line 65 to Col. 6 line 2 the patent teaches stainless steel heat treated coils in the convection section. The arrangement is “wrong” as the convection section seems to convey the feed from the radiant section to the desulphurization unit. However, the patent teaches the concept of using stainless steel tubing in the convection section. The patent does not differentiate between the various types of stainless steel.

U.S. Pat. No. 7,402,237 issued Jul. 22, 2008 to McCoy, assigned to ExxonMobil Chemicals Patents Inc., teaches vanadium pentoxide would also destroy the protective oxide layer on 304 stainless steel tubes to be used in the lower part of the convection section” (Col. 6, line 40 - 45). The reference clearly teaches using 304 stainless on the lower part of the convection section. The tubes in the convection do not appear to be coated. Again, the reference does not differentiate between the various types of stainless steel. This patent is addressing feedstocks that contain salts and or particulates.

U.S. Patent Application Publication No. 2011-0014372 published Jan. 20, 2011 in the name of Webber et al. The application, filed by Lyondell, was abandoned. Paragraphs 39 and 47 of the disclosure teach that conduit 5 may be stainless steel. From the figure, conduit 5 passes through the convection section of the furnace. Also, paragraphs 35 and 42 teach the line through the convection section and the radiant section may have the same internal coating. The published application teaches it is necessary that phosphorus is coated on the internal surface of the tubes or passes. Phosphorus can have several disadvantages. It is an environmental concern and needs continued replenishment. If it enters the grain structure, it lowers the grain boundary melting point weakening the metal. The passes or tubes used in the convection sections of the furnace of the present disclosure do not have internal coatings.

The present disclosure seeks to provide for the use of a high Cr/Ni alloy having less than about 70% iron, to have a significantly lower propensity to coke over conventional carbon steels and higher iron content stainless steels (e.g. 304).

SUMMARY OF THE INVENTION

In one embodiment, the present disclosure provides a convection section of a furnace to treat hydrocarbons, wherein not less than 50% of the coil length upstream from the outlet of the convection section, comprises less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from the group consisting of C, Mn, Si, P, and S.

A further embodiment provides a convection section of a furnace wherein not less than 75% of the coil length upstream from the outlet of the convection section have the composition as above.

A further embodiment provides the convection section of a furnace to treat hydrocarbons as above, wherein the coils further comprise 0.08 to 0.2 wt. % C; 1.5 to 2.5 wt. % Mn; 1.5 to 3 wt. % of Si, 0.04 to 0.05 wt. % P; and 0.25 to 0.35 wt. % of S.

A further embodiment provides the convection section of a furnace as above, wherein the hydrocarbons comprise one or more from C₂₋₄ paraffins.

A further embodiment provides a method to reduce fouling from the convection section of a furnace to treat hydrocarbons wherein not less than 50% of the coil length upstream from the outlet of the convection section comprise less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from the group consisting of C, Mn, Si, P, and S.

A further embodiment provides an above method wherein not less than 75% of the coil length upstream from the outlet of the convection section have the above composition.

A further embodiment provides an above method wherein the coils further comprise 0.08 to 0.2 wt. % C; 1.5 to 2.5 wt. % Mn; 1.5 to 3 wt. % of Si, 0.04 to 0.05 wt. % P; and 0.25 to 0.35 wt. % of S.

A further embodiment provides an above method wherein the hydrocarbons comprise one or more C₂₋₄ paraffins.

A further embodiment provides an above method wherein the hydrocarbon is ethane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an ethylene furnace.

FIG. 2 is a plot of flow coefficients for an ethylene cracker in which the carbon steel tubes in the convection section were replaced with stainless steel 310 (SS 310).

DETAILED DESCRIPTION Numbers Ranges

Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be understood as modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the properties that the present disclosure desires to obtain. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10; that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Because the disclosed numerical ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations.

All compositional ranges expressed herein are limited in total to and do not exceed 100 percent (volume percent or weight percent or mole percent) in practice. Where multiple components can be present in a composition, the sum of the maximum amounts of each component can exceed 100 percent, with the understanding that, and as those skilled in the art readily understand, the amounts of the components actually used will conform to the maximum of 100 percent.

FIG. 1 is a schematic diagram of a furnace (cracker) which may be used in any conventional application. One particularly useful application is in the cracking of chemical feedstocks, in many cases ethane, but the furnace could also be used with a naphtha feed or mixed feeds.

In a cracker 1, such as an ethylene cracker, the feed stock 2 enters a coil 3 typically passing through the exhaust area 4, typically referred to as the convection section. The feed is preheated in the convection section to a sub-cracking temperature. Typically, in a cracker, steam is fed to the convection section 4 through a parallel set of coils 6 to preheat it and then blend it with the feedstock stream. At the back end of the cracker is a quench unit 7 which cools the cracked gas and heats water in a heat exchanger 8 to generate steam. Steam from the heat exchanger 8 is fed through a separate set of coils 9 in the convection section 4 to further heat the steam for plant use.

The feed exits the convection section and typically travels through the furnace radiant section 5. In the furnace radiant section the coil may also be serpentine in configuration. There are a number of furnace configurations, such as, a single radiant section (fire box per the figure), parallel radiant sections (fire boxes), or it may comprise two radiant sections (fire boxes) in series, one cooler (cold box) and one hotter (hot box). However, both radiant sections typically share a common exhaust or convection section 4.

The feed is further heated in the furnace radiant sections by a number of burners 10, fed with a hydrocarbon fuel. In many cases, the fuel is a fluid, in particular instances a gas such as natural gas or natural gas mixture with other combustible gases, such as hydrogen. Low pressure combustion air is provided to burners 10 from a fan 11 through a duct system 12 or naturally aspirated through burner 10 registers. Each burner 10 has an associated variable air or oxygen flow controller 13 such as a damper or valve.

The control system for the furnace comprises a number of sensors or probes. In the arch 4 there is an oxygen probe 14 connected by electrical or optical cable 15 to a microprocessor 16. The microprocessor 16 is connected by an electrical or optical cable 17 to the fan 11. Increasing or decreasing the air fan speed or adjusting a set of fan louvers is used to control the amount of air supplied to the burners and, thus, dry oxygen in the exhaust gases.

Microprocessor 16 is connected by one or more electrical or optical cable(s) 18 to each air flow controller 13, and it can process the signals from the following probes:

i) an air pressure probe 19 which reads the pressure drop across the air or oxygen flow controller;

ii) a temperature probe 20, attached to the outlet piping of the radiant section and measuring the cracked gas temperature;

iii) a temperature probe 21, installed on the furnace wall proximate, typically within 1.5 meter about 5 feet from, the burner (e.g. the radiant section of the furnace wall);

iv) temperature probes 22 on sections of the coil or furnace tubes 3;

v) one or more (e.g. an array) remote sensor(s) 23 may be mounted on or adjacent a furnace wall, in a further embodiment.

While a significant amount of metallurgy has been developed to reduce coking in the radiant sections of the furnace, generally in the convection section the coils are made from carbon steel. As the furnace run length increases, coking from the tubes or passes in the convection section of the furnace can become a problem.

The issue of coking from the coils in the convection section of a furnace to treat hydrocarbons may be reduced by using a coil comprising less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from the group consisting of C, Mn, Si, P, and S such as for example a steel selected from the group consisting of stainless steel (SS) 310, 310S and 314 (SAE designation). In some embodiments the steel may further comprise 0.08 to 0.2 wt. % C; 1.5 to 2.5 wt. % Mn; 1.5 to 3 wt. % of Si, 0.04 to 0.05 wt % P; and 0.25 to 0.35 wt. % of S. The steel may further comprise a total of trace elements such as titanium or niobium, in a total amount of up to 2 wt. %.

The coil in the convection section may be made entirely of one of the above stainless steels. Composite coils could also be used comprising sections of the coil made using the above steel in the hotter areas of the convection section (closer to the outlet of the convection section) in combination with sections of the coil made from steels having a higher iron content such as carbon steel and low nickel (typically less than 12 wt. % Ni) austenitic stainless steels in the cooler areas of the convection section (farther from the feed outlet of the convection section). Typically wherein not less than 50%, in some embodiments not less than 75%, of the coil length (immediately) upstream from the feed outlet of the convection section comprise less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from the group consisting of C, Mn, Si, P, and S.

The coils or passes of the present disclosure do not have an internal coating of phosphorus.

The present disclosure also provides a method to reduce fouling from the convection section of a furnace to treat hydrocarbons wherein not less than 50% of the coil length from the outlet of the convection section comprising less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from the group consisting of C, Mn, Si, P, and S.

The convection section feed typically operates at temperatures from about 350° C. to about 800° C., typically 400° C. to about 800° C. The feed may be naphtha feed or derived from natural gas. Typically, the feed will be a C₂₋₄ paraffin. In some embodiments the feed predominantly comprises, more than about 85 vol. % ethane.

In some instances, coil treatment agents such as dimethyl disulphide may be added to the feed in amounts up to about 0.005 wt. %.

In one embodiment, the present disclosure provides a convection section of a furnace to treat hydrocarbons wherein not less than 50% of the coil length upstream from the feed outlet of the convection section, comprise less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from the group consisting of C, Mn, Si, P, and S.

In a further embodiment, there is provided a convection section of a furnace wherein not less than 75% of the coil length upstream from the feed outlet of the convection section have the composition of any other embodiment.

In a further embodiment there is provided the convection section of a furnace to treat hydrocarbons according to one or more other embodiments wherein the coils further comprise 0.08 to 0.2 wt. % C; 1.5 to 2.5 wt. % Mn; 1.5 to 3 wt. % of Si, 0.04 to 0.05 wt. % P; and 0.25 to 0.35 wt. % of S.

In a further embodiment, there is provided the convection section of a furnace according one or more other embodiments, wherein the hydrocarbons comprise one or more from C₂₋₄ paraffins.

In a further embodiment, there is provided'a method to reduce fouling in the convection section of a furnace to treat hydrocarbons wherein not less than 50% of the coil length from the feed outlet of the convection section comprising less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from the group consisting of C, Mn, Si, P, and S.

In a further embodiment, there is provided a method, wherein not less than 75% of the coil length from the feed outlet of the convection section have the composition of one or more other embodiments.

In a further embodiment, there is provided a method according to one or more other embodiments, wherein the coils further comprise 0.08 to 0.2 wt. % C; 1.5 to 2.5 wt. % Mn; 1.5 to 3 wt. % of Si, 0.04 to 0.05 wt. % P; and 0.25 to 0.35 wt. % of S.

In a further embodiment, there is provided a method according to one or more other embodiments, wherein the hydrocarbons comprise one or more from C₂₋₄ paraffins.

In a further embodiment, there is provided a method according to one or more embodiments, wherein the hydrocarbon is ethane.

The present disclosure is illustrated by the following non-limiting examples.

In a commercial ethylene cracker using ethane as a feed having a high nickel/chrome stainless steel in the furnace tubes in the radiant section and carbon steel in the convection section, the better run times between decoking ranges from 134 days to 238 days. The convection section of the furnace was rebuilt and the carbon steel tubes were replaced with SS310 tubes. The run time was extended to 406 days. The furnace run was terminated due to a full plant outage and not because the furnace required an outage. What is important to note is that after the rebuild the flow coefficient for the best run was significantly extended and also the profile for the flow coefficient was very flat compared to the prior runs with carbon steel.

This is shown in FIG. 2, which shows that using stainless steel 310 significantly reduces coking from the convection section of a cracker. 

What is claimed is:
 1. A convection section of a furnace to treat hydrocarbons wherein not less than 50% of the coil length upstream from the feed outlet of the convection section, comprise less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from C, Mn, Si, P, and S.
 2. A convection section of a furnace wherein not less than 75% of the coil length upstream from the feed outlet of the convection section have the composition of claim
 1. 3. The convection section of a furnace to treat hydrocarbons according to claim 1, wherein the coils further comprise 0.08 to 0.2 wt. % C; 1.5 to 2.5 wt. % Mn; 1.5 to 3 wt % of Si, 0.04 to 0.05 wt. % P; and 0.25 to 0.35 wt. % of S.
 4. The convection section of a furnace according to claim 3, wherein the hydrocarbons comprise one or more from C₂-C₄ paraffins.
 5. A method to reduce fouling in the convection section of a furnace to treat hydrocarbons wherein not less than 50% of the coil length from the feed outlet of the convection section comprises less than 66 wt. % of Fe and the balance a mixture of 23 to 26 wt. % of Cr; 19 to 22 wt. % of Ni; and one or more components selected from C, Mn, Si, P, and S.
 6. The method according to claim 5, wherein not less than 75% of the coil length from the feed outlet of the convection section have the composition of claim
 5. 7. The method according to claim 6, wherein the coils further comprise 0.08 to 0.2 wt. % C; 1.5 to 2.5 wt. % Mn; 1.5 to 3 wt. % of Si, 0.04 to 0.05 wt. % P; and 0.25 to 0.35 wt. % of S.
 8. The method according to claim 6, wherein the hydrocarbons comprise one or more from C₂-C₄ paraffins.
 9. The method according to claim 8, wherein the hydrocarbon is ethane. 